In telephone systems, in particular, an automatic exchange may serve any number of incoming and outgoing PCM lines ranging from a few hundred in the case of a local exchange to several hundreds of thousands in the case of transit exchanges. It is therefore desirable to provide flexible switching networks essentially built up from integrated modular units whose number can be enlarged as needed. Such flexibility also requires a decentralized control of the switching operations of the individual units and of ancillary equipment serving to detect possible malfunctions; should such a malfunction occur, the number of lines affected by any partial deactivation of the network ought to be minimized. Moreover, the network should be of essentially nonblocking character to optimize its traffic-handling capacity and, of course, transit times as well as power consumption should also be at a minimum.
Various structures have already been proposed with a view to fulfilling at least some of these requirements. Thus, for example, FIG. 11 of U.S. Pat. No. 4,093,827 discloses a network with a group of three switching stages responding to routing instructions from a lower-ranking control unit individual to that group, bracketed by input and output circuits under the control of a higher-ranking central unit.